Download Phys 102 – Lecture 28

Phys 102 – Lecture 28
Life, the universe, and everything
1
Today we will...
• Learn about the building blocks of matter & fundamental forces
Quarks and leptons
Exchange particle (“gauge bosons”)
• Learn about the Big Bang theory
Hubble law & the expansion of the universe
The early universe
Unification of forces
Phys. 102, Lecture 16, Slide 2
Fundamental particles
Are the electron, proton, and neutron the fundamental building blocks of matter? Evidence says NO for proton & neutron
Particle “zoo”
Hundreds of particles identified in particle accelerator experiments
“sigma” Σ0, Σ+, Σ–
“pion” π0, π+, π–
“xi” Ξ0, Ξ–
etc...
“kaon” κ0, κ+
Neutron magnetic dipole moment

S

μn
neutron
Neutron has spin ½, is electrically neutral, yet has a magnetic dipole moment!
Indicates these are composite particles
Phys. 102, Lecture 28, Slide 3
Quarks
“Three quarks for Muster Mark” Finnegan’s Wake, James Joyce
Neutrons and protons are composite particles
Discovered in 1968
“Flavors”
Quark
up (u)
down (d)
Hadrons are particles composed of quarks
1 proton = uud
u
Charge
 23 e
 13 e
u
d
1 neutron = udd
d
u
e
d
0
Phys. 102, Lecture 28, Slide 4
ACT: Beta decay
Last lecture, we saw that β– decay involves converting a neutron into a proton. 
Th 
234
90
234
91
Pa 
e  00 e
0 
1
1
0
n  11 p 
e  00 e
0 
1
How could this decay be described in terms of quarks?
A. A d converts to a u
B. A u converts to a d
C. A d converts to an e–
Phys. 102, Lecture 28, Slide 5
ACT: Hadrons & quarks
The Δ– is an exotic hadron with charge –e. ?
?
?
e
What could the quark makeup of this particle be?
A. uuu
B. ddd
C. an e– & νe
Phys. 102, Lecture 28, Slide 6
Building blocks of matter
Ordinary matter is made of u, d (quarks), e and νe (leptons) Generation
1
3
2
‘74
‘95
Hadrons (ex: n, p) are composite particles made of quarks
‘74
‘77
‘75
‘00
Mass
Quarks and leptons (ex: e–) are believed to be the elementary particles
There is a corresponding anti‐
particles for each elementary particles! Same m, opposite q
Phys. 102, Lecture 28, Slide 7
ACT: Quarks
In Lect. 26 we saw that two electrons cannot be in the same state (i.e. have the same quantum numbers).
Can two quarks be in the same state?
A. Yes
B. No
Phys. 102, Lecture 28, Slide 8
4 Fundamental forces of Nature
Gravitational force (solar system, galaxies)
Electromagnetic force (atoms, molecules)
Strong force (atomic nuclei)
Weak force (radioactive decay)
<
Gravitational
weakest
<
Weak
Electromagnetic
<
Strong
strongest
Particle physics view of forces
Matter interacts through exchange of mediator or exchange
particles
Ex: electromagnetic exchange particle is the photon!
Photon is virtual and cannot be observed
Time
γ
Coulomb repulsion
–
–
Summing over all the possible ways photon can be exchanged leads to Coulomb’s law
Space
“Feynman diagram”
Phys. 102, Lecture 28, Slide 10
The “Standard Model”
Exchange particles for are known as gauge bosons
‘79
Strong force
Electromagnetic force
‘83
β– decay ‘83
Weak force
Only force that can change quark flavor
What about gravity?
Phys. 102, Lecture 28, Slide 11
The Higgs boson
Higgs boson gives elementary particles their masses
‘13
The more massive the particle, the more it interacts with the Higgs boson
Massless photon
γ
Massive electron
–
Phys. 102, Lecture 28, Slide 12
ACT: Fundamental forces
Which of the following particles can interact via the electromagnetic force? A.
B.
C.
D.
E.
A muon
An up quark
A strange quark
All of the above
None of the above
Phys. 102, Lecture 28, Slide 13
The expansion of the universe
Astronomers observed that all celestial bodies are receding
from us. Therefore, the universe is expanding!
Phys. 102, Lecture 28, Slide 14
ACT: Doppler effect
Recall Lect. 15 The wavelength λobs observed on earth from the spaceship is
Earth
λemit
v
A. Larger than λemit
B. The same as λemit
C. Smaller than λemit
Phys. 102, Lecture 28, Slide 15
ACT: Hubble law
More distant celestial objects recede from us faster
Recessional velocity
v  H 0d
Distance to object
“Hubble” constant
The light spectrum of a stationary galaxy is given by: Recall Lect. 25 400
500
600
700
λ (nm)
Which of the following belongs to the most distant galaxy?
A.
B.
C.
Phys. 102, Lecture 28, Slide 16
The Big Bang
All celestial bodies are receding from us and each other, so universe must be expanding...
DEMO
Distances increase in every direction
Past
Future
Today
d
Assuming a constant rate of expansion: v   H 0 d
t
1
1 s  Mpc
km
1
yr

tuniverse 
3  1019
 14 Gyr
70
km
Mpc 60  60  24  365 s
H0
1 “Megaparsec”= 1 Mpc
= 3×1019
km
Best estimate is 13.8 Gyr
Phys. 102, Lecture 28, Slide 17
A journey back in time...
Early universe was smaller, more dense, & hotter
Atoms are ionized & universe opaque
Matter coalesces by gravity into first stars & galaxies First stable nuclei form
First hadrons form
Universe neutral atoms, “dark ages”
Universe quark‐gluon plasma
1 μs 1 s 3 min 20 min 380 kyr 8 Myr
13.8 Gyr
TIME
ENERGY
Phys. 102, Lecture 28, Slide 18
Unification
At high energies, fundamental forces begin to look the same “Grand Unified Theories”
(~1016 GeV)
“Theory of Everything”
(~1019
GeV)
Electroweak Theory (~102 GeV) 1979, 1983
String theory, quantum gravity, etc.
10–43 s
10–35 s
10–11 s
TIME
ENERGY
Phys. 102, Lecture 28, Slide 19
Some unsolved problems
What is dark matter?
We cannot detect most of the matter in the universe. It is “dark”.
What is the nature of dark energy?
The expansion of the universe is accelerating. A “dark energy” is driving this acceleration
Why is there more matter than antimatter?
The universe is made up mostly of matter
Can the fundamental forces be unified?
There is no unified model of electroweak & strong force, nor a quantum theory of gravity Phys. 102, Lecture 28, Slide 20